Blood: Structure, Function, and Clinical Relevance

Blood: Structure, Function, and Clinical Relevance

Objectives

This section outlines the key learning goals for understanding blood in human anatomy and physiology:

• List the functions and components of blood.

• Identify erythrocytes, leukocytes (with types), and platelets.

• Explain the significance of hematocrit and blood factors such as ABO and Rh.

• Describe the structure and function of plasma and formed elements.

• Discuss blood groups, agglutination, and transfusion reactions.

• Describe the safety and clinical implications of blood typing.

Circulatory System Overview

General Functions

The circulatory system is essential for transporting substances throughout the body and maintaining homeostasis.

• Consists of the heart (pump), blood vessels, and blood.

• Picks up nutrients from the digestive system.

• Exchanges gases with the respiratory system.

• Delivers nutrients and O2 to every cell.

• Carries CO2 to lungs for removal.

• Carries wastes and excess water to urinary system.

• Carries metabolic wastes to liver for removal.

• Helps regulate body temperature.

The Composition and Functions of Blood

Blood as a Connective Tissue

Blood is a specialized connective tissue composed of cells and cell fragments suspended in plasma.

• Specialized cells and cell fragments

• Watery solution of ions, molecules

Primary functions:

1. Transportation of nutrients, wastes, hormones

2. Regulation of body temperature, water volume, pH

3. Defense against infections and bleeding

Blood Components

• Plasma (extracellular matrix): ~55% of whole blood

  • Water

  • Electrolytes (ions)

  • Proteins (albumin, globulins, clotting proteins)

  • Hormones

  • Gases

  • Nutrients and wastes

• Formed elements: ~45% of whole blood

  • Red blood cells (erythrocytes)

  • White blood cells (leukocytes)

  • Platelets

Blood Plasma

Composition

Blood plasma is the liquid component of blood, making up about 90% water and containing many dissolved substances.

• Nutrients

• Salts (electrolytes)

• Respiratory gases

• Hormones

• Plasma proteins

• Waste products

Plasma Proteins

• Most abundant solutes in plasma

• Most are made by the liver

• Types:

  • Albumin: regulates osmotic pressure

  • Clotting proteins: help stem blood loss when a blood vessel is injured

  • Antibodies: help protect the body from pathogens

Formed Elements

Types

• Erythrocytes (RBCs)

• Leukocytes (WBCs)

• Platelets (cell fragments)

Erythrocytes (Red Blood Cells)

• Main function: carry oxygen

• Contain hemoglobin (iron-containing protein)

• Essentially bags of hemoglobin; anucleate (no nucleus)

• Normal count: 5 million RBCs per cubic millimeter of blood

Hemoglobin

• Binds strongly, but reversibly, to oxygen

• Responsible for oxygen transport

Leukocytes (White Blood Cells)

• Crucial in defense against disease

• Complete cells with nucleus and organelles

• Move in and out of blood vessels (diapedesis)

• Move by amoeboid motion

• Respond to chemicals released by damaged tissues (positive chemotaxis)

• Normal count: 4,000 to 10,800 WBCs per cubic millimeter of blood

Types of Leukocytes

• Granulocytes:

  • Neutrophils

  • Eosinophils

  • Basophils

• Agranulocytes:

  • Lymphocytes

  • Monocytes

Granulocytes

Type	Features	Function	Normal Count
Neutrophils	Pale pink cytoplasm, fine granules	Phagocytosis at infection sites	3,000–7,000/mm3 (40–70%)
Eosinophils	Red, coarse granules; bilobed nucleus	Fight parasitic worms, allergy attacks	100–400/mm3 (1–4%)
Basophils	Large blue-purple granules; S-shaped nucleus	Release histamine (vasodilator), contain heparin (anticoagulant)	20–50/mm3 (<1%)

Agranulocytes

Type	Features	Function	Normal Count
Lymphocytes	Pale blue cytoplasm, dark nucleus	Immune response, produce antibodies	1,500–3,000/mm3 (20–45%)
Monocytes	Largest WBC, kidney-shaped nucleus	Phagocytosis, fight chronic infection	100–700/mm3 (4–8%)

Platelets

• Derived from ruptured multinucleate cells (megakaryocytes)

• Needed for the clotting process

• Normal count: 150,000 to 400,000 per cubic millimeter of blood

Human Blood Groups

Antigens and Antibodies

• Blood contains genetically determined proteins called antigens (cell surface proteins).

• Antigens allow the body to recognize self vs. non-self.

• Antibodies are the recognizers of non-self antigens.

• Blood is typed using antibodies that cause agglutination (clumping) and lysis.

ABO Blood Groups

• Based on the presence or absence of two antigens: Type A and Type B.

• Lack of these antigens is called Type O.

• Type AB: both antigens present (universal recipient).

• Type O: neither antigen present (universal donor).

Blood Type Compatibility

Recipient Blood Type	Can Receive From	Notes
AB	A, B, AB, O	Universal recipient
B	B, O
A	A, O
O	O	Universal donor

Rh Blood Groups

• Named for the presence or absence of one of eight Rh antigens (agglutinogen D).

• Most Americans are Rh-positive.

• Problems can occur in mixing Rh+ blood into a body with Rh- blood.

• Hemolysis does not occur with first transfusion; takes time to form antibodies.

• Subsequent transfusions can cause immune reactions.

Rh Dangers During Pregnancy

• Danger occurs when mother is Rh- and father is Rh+, child inherits Rh+ factor.

• RhoGAM shot can prevent buildup of anti-Rh antibodies in mother's blood.

• First pregnancy usually proceeds without issue; sensitization occurs after first exposure.

• Second pregnancy: mother's immune system attacks Rh+ blood of fetus (hemolytic disease of the newborn).

Blood Typing

Principles

• Blood samples are mixed with anti-A, anti-B, and anti-D (Rh) serum.

• Agglutination or lack thereof identifies blood type.

• Typing for ABO and Rh factors is done similarly.

• Cross-matching tests for agglutination of donor RBCs by recipient's serum and vice versa.

Hematocrit

Definition and Calculation

The hematocrit is the percentage of blood volume occupied by erythrocytes (RBCs).

• Relative measure of O2 carrying capacity of blood.

• Normal range for men: 43–49%

• Normal range for women: 37–43%

Calculation:

• Measures the entire volume of blood and the column of RBCs.

• Formula: $\text{Hematocrit} = \frac{\text{Height of RBC column}}{\text{Total height of blood column}} \times 100\%$

Clinical Relevance

• Unusual hematocrit may indicate anemia or polycythemia.

• Anemia: low O2 carrying capacity; many types:

  • Hemorrhagic anemia: rapid blood loss

  • Iron-deficiency anemia: inadequate iron intake or absorption

  • Pernicious anemia: autoimmune disease, poor vitamin B12 absorption

  • Renal anemia: lack of erythropoietin

  • Aplastic anemia: destruction/inhibition of red marrow

• Polycythemia: excess RBCs, often due to bone marrow disorders

Example

If the height of the RBC column is 124 mm and the total height of the blood column is 255 mm:

$\text{Hematocrit} = \frac{124}{255} \times 100\% \approx 49\%$

Additional info: Hematocrit is a key diagnostic tool for evaluating anemia and polycythemia, and is routinely measured in clinical settings.